Weissbier, I've got the exact same cooler - just not at that price. Good find. It's done pretty well for 1 10-gallon and 2 5 gallon batches. I built a cpvc manifold which seems to work great. I'm fine-tuning my efficiency right now, but I don't think the cooler is affecting my lower numbers, more my mash thickness, and possibly the crush.
It does tend to lose some heat over the mash, so expanding foam in the lid would be good. I used a blanket over the top last time and I only lost 2 degrees over an hour. When you build your valve, make sure you include a large washer (stainless) on the inner wall. I don't have one right now, and I get really nervous cranking the handle, as the side flexes with the torque as I do so. Darthbeer is right - there is no insulation in that spot, just a thin plastic wall.